Slide 11.1: An overview of the implementation

An Overview of the Implementation

The figure below shows an abstract view of the MIPS subset implementation:

All instructions start by using the program counter to supply the instruction address to the instruction memory.

After the instruction is fetched, the register operands used by an instruction are specified by fields of that instruction.

Once the registers have been fetched, they can be operated on to find a memory address, to compute an arithmetic result, or a compare.
- If the instruction is an arithmetic-logical instruction, the result from the ALU must be written to a register such as
```
   add  $t1, $t2, $t3       # $t1 = $t2 + $t3
```
- If the operation is a load/store, the ALU result is used as an address such as
```
   lw  $t1, offset($t2)     # $t1 = mem[[$t2]+offset]
```
- Branches require the use of the ALU output to determine the next instruction address such as
```
   beq  $t1, $t2, offset    # branch if $t1 == $t2
```

The thick lines interconnecting the functional units represent buses, which consist of multiple signals. The arrows are used to show how information flows.